It is conventional in the art to produce wood trusses that support the roofs of sheds, wood beams and even wood boards in the form of laminated components by a process wherein a plurality of wood lamellae, or wood plies, are bonded together in a sandwich-like array or structure. The wood lamellae are thin boards having a thickness ranging from a few millimeters to over one centimeter, and having lengths of up to several meters. The advantage of using such laminated structures is that the presence of a number of bonded wood plies compensates for the natural points of weaknesses in wood. The progressive overlapping bonding of the wood plies also allows the manufacture a wood structure having dimensions that exceed those dictated by the size of a natural tree trunk. Such laminated wood structures generally exhibit a substantially greater strength than non-bonded solid wood products.
It also has been demonstrated that wood plies or lamellae can be produced by the chipless cutting of squared timber with one or more blades or knives. For example, such a method is described in German Patent publication DE-OS 37 02 909A, corresponding to the U.S. Pat. No. 4,825,917, incorporated herein by reference. An important advantage in the chipless production of wood lamellae or plies is the substantial saving in raw material, because, when wood plies are produced by cutting with saws, 40% by weight or more, depending upon the thickness of the wood plies, of the timber starting material is lost as sawdust.
In the chipless production of wood plies from squared timber using a blade, the wood plies are deflected from the squared timber at the angle of the blade edge because of the unavoidable transverse dimension of the blade. This direct lateral deflection of the wood ply as the ply is separated from the squared timber leads not only to a generally biaxial warping of the wood ply, that subsequently can be substantially corrected by straightening devices, but also affects the surface structure of the cut wood ply.
In this respect, it has been discovered that the side of the wood ply, or lamella, that contacts the edge of the blade during the separation of the lamella from the squared timber, and here and hereinafter referred to as the "knife side" of the lamella, has a relatively open surface structure that occasionally has loose wood fibers projecting from the lamella. Therefore, the knife side is also referred to as the open side of the wood lamella. In contrast, the side of the wood lamella opposite the knife side, referred to here and hereinafter as the "opposite side", only has shallow depressions and is generally smooth. Accordingly, the opposite side of the lamella also is referred to as the closed side of the lamella. The differing surface properties between the knife side and the opposite side of the lamella can be significant in regard to wood structures that have a visible external surface that should be as smooth and even as possible.
Therefore, one aspect of the present invention is to provide a method of manufacturing a multi-ply solid wood structure that optimizes the use of raw material through a chipless cutting of wood plies, and that yields multi-ply wood structures that demonstrate maximum strength. In accordance with the present invention, it has been discovered that the fracture resistance under flexure of a laminated component made of cut wood lamellae produced by a blade cut can be optimized if the opposite side of each wood lamellae comprising the multi-ply wood structure is directed away from the outer surface of the multi-ply wood structure subjected to a tension load.
In accordance with a preferred embodiment of the invention, the strength of the multi-ply wood structure can be increased by arranging the wood lamella or plies such that the opposite side of each wood ply is directed towards one and the same external surface of the multi-ply wood structure. In order to best utilize the strength properties of the multi-ply wood structure, it is necessary that the external surface to which the opposite side of each wood ply is directed is positioned as the external surface remote from the acting flexing force. For reasons of appearance, it may be desirable for the lamella or ply on the opposing external surface of the multi-ply wood structure to be arranged such that opposite side of the wood ply forms the opposing external surface.
Aligning the opposite side of essentially all the wood lamellae in a multi-ply solid wood structure to be directed toward one and the same external surface of the multi-ply wood structure has a disadvantage in that the maximum load carrying capacity of such a wood structure is dependent on the alignment of the wood structure in relation to other structures joined to it. The strength properties of the solid wood structure are optimized when the opposite side of all the lamellae face one outer surface. But, then the enhanced strength properties of the solid wood structure are realized only when the structure is installed correctly, generally with the opposite side of the lamellae facing downward under the assumption that a load or bending (flexing) force that is applied from above.
If such a wood structure is installed upside down, the strength properties of the structure would be decreased. The problem of correct installation is overcome when the structure has, for example, a cross section in the shape of a parallelogram, as it is common with some roof-bearing girders, then no mispositioning can occur.
It is different, however, when the wood structure has a symmetrical cross section. Then the outer surfaces of such a structure would have to be marked in order to install the structure in the correct position. Normally, and more particularly in the case of boards and beams having a rectangular cross-section, the wood lamellae therefore are preferably aligned in the multi-ply structure such that their opposites sides are, when considered from a center plane of the multi-ply structure running parallel to the outer surfaces thereof, respectively directed toward the outer surfaces. Then, the alignment of the multi-ply structure in relation to the rest of the structure is not significant. In this embodiment, wherein the opposite sides of the lamellae all face away from the center plane, the installation of the wood structure is independent with respect to its upper or lower surface. This embodiment, however, does not possess the strength properties of the embodiment wherein the opposite side of all the lamellae face to one outer surface, provided such structure is correctly installed.
In addition to the systematic alignment of the opposite side of the wood lamellae that are joined to provide a multi-ply wood structure as discussed above, another significant factor influencing the strength of the wood structure is the production of the lamellae themselves. The method of preparing the lamellae ensures inter alia that the lamellae can be joined together into a compact block having a minimum number of gaps. Minimizing the number of gaps between the lamellae generally is accomplished by drying the lamellae after separation from the squared timber while the lamellae are subjected to a sufficient load to correct the warp that arises due to the separation process. The lamellae usually then are trimmed along their longitudinal edges, and can, in addition, be lightly sanded on their principal surfaces in order to more particularly remove any projecting wood fibers present on the knife side of the lamellae. Furthermore, it is known in the art to sand the finished multi-ply wood structure to provide a desired surface smoothness and appearance.